StackController has no visual indication of the active/current tab

When focus is within a stack container page there is no indication of which tab the content is associated with. I guess the StackController? should indicate focus similar to how TabController? indicates it. I think the button has a "selected" state, IIRC.

If we do add visual indication, it needs to also work in high contrast mode. For the tab container, the active tab is indicated with a dashed border in high contrast mode and using a different background color in "standard" mode.